Yeah, Joe W's answer is probably correct because the latter article I was reading eventually gets to
The closest Congress came to addressing the issue was in 1954, when a large Senate majority approved a constitutional amendment that would have forced all federal judges to retire at 75. Days later, however, the Supreme Court handed down its ruling in Brown v. Board of Education, and the nation’s attention shifted to the fight over desegregation.
I haven't dug up the details on that failed proposal (as to whether in included anything else), but it seems that merely adding an age limit would require a constitutional amendment.
As another (very long) paper recounts, it was in fact the 2nd time such a proposal was made. There was a prior one during FDR's term, distinct from his court packing threats, and in fact introduced (by several Senators) as a moderate alternative to FDR's packing plan, but FDR refused to consider the former and age-limits were not put up to a vote then.
Also, that paper says
Historical remembrance that this effort ever occurred appears to be almost wholly lacking from the entire post-1954 historiography of the Supreme Court [...]
The 1946-1954 campaign for age limits was initially led by Edwin A. Falk, and was thoroughly discussed by the ABA, which then forwarded it to closely affiliated Senators. Falk's proposals were actually aimed at preventing an FDR-type packing threat, so he wanted an amendment to fix the number of justices (at 9) among other things (which included an retirement age). Eventually this led to an ABA resolution:
the committee resolved to return its recommendations to the House of Delegates at its next meeting in late February of 1950, and at that time the House officially approved both a proposed amendment fixing the size of the Court at nine and compelling retirement at age seventy-five and a separate proposal rendering individuals ineligible for the presidency or vice-presidency within five years of having served on the Court.
In mid-May of 1952, without even a prefatory speech on the
Senate floor or any other attendant publicity, [Senator] Butler introduced a resolution detailing an amendment that would fix the Court's size at nine, mandate retirement at age seventy-five, and insulate the Court's appellate jurisdiction over constitutional cases from any congressional alteration.
So it wasn't a simple age-limit proposal.
An identical companion measure, House Joint Resolution 194, was soon introduced by Representative Edward T. Miller of Maryland, but not until early 1954 did Butler's measure receive a hearing before the Senate Subcommittee on Constitutional Amendments.
And it (Senate Joint Resolution 44) quickly passed the Senate in a rather quiet moment:
within minutes the resolution was put to a vote and passed by a tally of fifty-eight to nineteen (with nineteen absent and not voting), well more than the two-thirds margin needed for approval. [...]
Three days after the Senate approval, the New York Times published a cautionary editorial echoing Senator Hennings's concerns.
The amendment had been approved with "virtually no notice by the American public. No matter how meritorious such an amendment may be-and in this case we think it is meritorious-this is not the right way for the Senate to pass upon a substantial change in the nation's fundamental law."
[...] in late June a House Judiciary subcommittee accorded the proponents a one-day hearing. Senator Butler testified first, explaining how the Senate had adopted Attorney General Brownell's suggestion that the mandatory retirement provision apply to all federal judges and how he had jettisoned the five-year ban regarding presidential and vice-presidential candidacies. "[R]ather than risk losing the whole amendment, I consented to having this section dropped," he revealed.'
As that paper also recounts, the ultimate irony was that the amendment had been supported by precisely the group that was then incessed by Brown:
Most illustrative of how the ABA's congressional world was so
completely upended by the Court's mid-1950s rulings was the utterly inverted stance of Maryland Senator John Marshall Butler toward the Court he had been so eager and enthusiastic to protect from congressional or executive encroachment in 1953 and 1954. Within just three years Butler was transformed into "an outspoken critic of the Warren Court," and in early 1958 Butler introduced a bill that would have removed from the Court's appellate jurisdiction bar admissions cases
like Schware and Konigsberg-precisely the sort of legislative intrusion his 1953-54 constitutional amendment had been designed to prohibit. Butler's bill failed, but as Court scholar Walter Murphy noted several years later, the Maryland Senator "sorely regretted his earlier efforts to protect the Court's jurisdiction."
That paper also notes that there is widespread, but not quite universal consensus that impeachment is the only constitutional removal mechanism of federal justices. It then details the years-long campaign of Senator Nunn to pass a law (rather than constitutional amendment) that would have allowed justices to be removed for mental incapacity by a majority of the Judicial Conference. All of these attempts failed, although a watered down version did pass the Senate (S 1423) "on September 7, 1978, by a margin of forty-three to thirty-two" but was then not considered by the House of Representatives. It was further watered down in 1979 (S 1873) by removing Supreme justices from scope and more or less reduces the role of the Conference to recommending impeachment to the House of Representatives in such cases; this (much watered-down version) did pass into law as the Judicial Councils Reform and Judicial Conduct and Disability Act of 1980.